Hard disk drives store information on magnetic disks. Typically, the information is transferred to and from concentric tracks of the disk that are divided into servo sectors and data sectors. Information is written to or read from the disk by a transducer, or head, that is mounted on an actuator arm that positions the transducer over the disk in a predetermined location. Accordingly, movement of the actuator arm allows the transducer to access the different tracks of the disk. The disk is rotated by a spindle motor at a high speed, allowing the transducer to access different sectors within each track on the disk.
A voice coil motor (VCM), in combination with a servo control system, is usually employed to position the actuator arm. The servo control system generally performs the function of seek control and track following. The seek function is initiated when a command is issued to read data from or write data to a target track on the disk. Once the transducer has been positioned sufficiently close to the target track by the seek function of the servo control system, the track following function of the control system maintains the transducer on the target track until the desired data transfer is completed.
Typically, the transducer will oscillate about the center line of the target track for a period of time following the transition of the servo control system from the seek mode to the track following mode. These off-track displacements, or post-seek oscillations, are due, at least in part, to mechanical vibrations generated by the components of the disk drive during the seek and/or tracking operation. In addition, while in the track following mode, adjustments to the position of the transducer with respect to the center line of the target track are often required due to mechanical vibrations. Such adjustments are required to correct drift in the position of the transducer relative to the target track. The precise control of the position of the transducer relative to a target track has become increasingly important as track densities (or tracks per radial inch—TPI) in disk drives have increased. More specifically, the number of tracks included on a disk, i.e., the greater the TPI, translates to higher data storage capability. However, the increased number of tracks means that there is a more stringent requirement that the transducer stay on track for both reading and writing purposes as the separation distance between adjacent tracks decreases.
A measure of how far the transducer is off target is termed “Track Misregistration” (TMR). It can be measured in distance (e.g. microns) or as a percentage of track pitch. TMR is also referred to as off track or track following errors.
Disk drives generally utilize a voice coil that interacts with a pair of permanent magnets. Generally, the coil is secured by two prongs that form the yoke of the actuator assembly. Electric current in the voice coil interacts with the magnetic field generated by the permanent magnets, thus allowing the head of the actuator assembly to be selectively positioned by variance of the current.
The actuator assembly, including the voice coil and yoke, actuator arm, suspension and slider, generates vibrational loads that impair the ability of the actuator assembly to position and maintain the transducer over a desired track. To account for vibrational loads, during the design phase, the amount of vibration from the assembled components are assigned a budget, that must not exceed a predetermined level of generated vibration thus minimizing TMR and post seek oscillation errors. These budgets are based upon vibrations originating from a number of sources and take on various forms including, but not limited to, electrical noise torque, whirl, arm mode, drum mode, high frequency turbulence, disk vibration, aerodynamic torque, and external vibration or seek settle. More specifically, many of the vibrational loads are generated by the different modes of vibrational motion generated by the components of the actuator assembly. Minute vibrational loads that emanate from the aerodynamic loading of the disk and/or actuator moving through the air inside the disk drive housing may also affect TMR and post seek oscillations. Thus, it is important for designers of disk drives to reduce the individual sources of vibrational loads that influence positioning of the transducer to produce a disk drive with lower vibrational loading such that the servo control system may better compensate for TMR and post seek oscillations.
In addition to the post seek oscillations generated by the components of the actuator assembly, post seek oscillations are also caused by the acceleration and deceleration of actuator arm movement as it moves from one track to an intended track. Other sources of post seek oscillation will be apparent to one skilled in the art, such as those from the interaction between various components such as the bearing and the actuator when the actuator slows or stops the flex circuit and the actuator, the rotation of the disk, the interaction of the voice coil motor with the driver, etc.
The negative effects of TMR and post seek oscillations are most easily described by a brief discussion of track pitch. The distance between two concentric tracks of a disk is known as track pitch, which decreases as TPI increases. For example, a disk with 100,000 TPI has generally a track pitch budget of 0.254 microns, (10 millionths of an inch), wherein a disk with a 150,000 TPI has a track pitch of about 0.17 microns (approximately 7 millionths of an inch). As described above, each vibration-generating component of a disk drive has a budget that contributes to the maximum allowable TMR that are correctable by the servo control system. That is, vibrational induced oscillations of the transducer must be maintained at or below a level where the servo controller can effectively counteract the movement and control the position of the transducer. This level is predetermined in the design of a disk drive. Returning now to the example in which a 100,000 TPI disk drive was described, vibrations generated by the voice coil may cause transducer displacements of about plus or minus 0.32 microns (1.25 millionths of an inch) from the intended track centerline. The servo controller of the actuator arm is then tasked to reduce this movement generated by the coil. For this example, it is assumed that the servo controller is capable of reducing the oscillations to 0.050 micron, or plus or minus 0.025 micron off its intended path. Thus, the vibrations generated by the voice coil take up 10% of the track pitch based off-track budget of the servo control system (0.025 micron÷0.25 microns). When the TPI of the disk is increased to 150,000, and the same servo controller is used (which reduces the effect of the oscillations down to plus and minus 0.025 micron), vibrations generated by the voice coil now equal about 15% of the total budget (0.25 micron÷1.7 microns). This translates to perhaps a 5% increase of budget consumption due to vibrations generated by the voice coil when the TPI of the disk drive is increased fifty percent. If the goal of the designers is to maintain the TMR budget generated by the voice coil in the 10% of track range despite the increase in TPI, additional attenuation techniques are required.
Alterations of voice coil resonance frequencies have been shown to reduce head deflections. A major driver in this vibration source is the coil resonant frequency. It has been shown that by stiffening the coil to raise its resonance frequency, its interaction with the head of the actuator arm is reduced. One method to stiffen the coil, as described by WO 01/26098 to Heath (hereinafter “Heath”), is to add a pair of plates to the actuator assembly in the area of the voice coil and arms of the yoke. Heath generally describes two embodiments. One positions plates overlying and extending beyond both the upper and lower surfaces of the voice coil to the outer edge of the arms forming the yoke, thus creating a sandwich structure that stiffens the voice coil. The second removes the yoke arms altogether and only uses the pair of plates to retain the voice coil. These embodiments are shown in FIGS. 4A and 4B. One drawback with these approaches is that both plates are located within the magnetic gap between the permanent magnets and the voice coil, such that magnetic field interactions are decreased thereby affecting the efficiency of the voice coil motor. Another drawback with this structure is that the addition of the plates increases the overall height of the voice coil assembly, thereby increasing the height requirements of the overall voice coil motor, e.g., the distance separating the permanent magnets, and increasing the height of the disk drive housing. To compensate and maintain existing disk drive housing height, Heath discloses decreasing the height or thickness of voice coil and yoke arms to accommodate the height of the plates. However, this alteration has potential adverse side effects. Decreasing the height of the voice coil will necessarily remove windings such that the magnetic field intensity of the trimmed voice coil will be reduced thereby decreasing the efficiency of the voice coil motor. Further, Heath in some instances utilizes plates made from stainless steel. This increases the chance of eddy currents being generated within the plates, also affecting efficiency of the voice coil motor.
Another drawback of Heath is that the added weight of the plates influences the efficiency of the servo mechanisms to position the heads. Heath has proposed a method of decreasing this effect by integrating holes into the plates to remove weight. These holes generally reduce the stiffness of the plates, thus making them less effective as dampeners. In addition, Heath proposes reducing the thickness and/or length of the yoke arms, or completely removing the yoke arms, to compensate for the added weight of the skins. The stiffness of the yoke arms is generally related to their thickness. Reducing the arm thickness will create a member that is more prone to vibration. Removing the arms completely also drastically alters stiffness of the overall structure. Decreasing the length of the yoke arms may also adversely affect the disk drive crash stop. In many instances, the crash stop interacts with the yoke arms to prevent the actuator assembly from rotating past a certain point. Alteration of the yoke arms would likely require alteration to the crash stop components, thereby requiring a new design to replace one that now works well, and necessarily creating additional manufacturing steps.
It is also known in the art to wind the wires of the voice coil around a plastic bobbin. Bobbins allow the inner diameter of the voice coil to be precisely controlled. However, as used in the art today, the plastic bobbins offer, at best, a modest increase in stiffness and a related modest reduction of the sway mode, but they do not provide any reduction in bending and torsional loads in any significant manner. Conversely plastic bobbins generally increase the effect of coil bending and torsion loading. In addition, over time the bobbin tends to separate from the coil, thereby degrading performance.